2G Systems and Networks

2G Systems and Networks
The 2G systems evolved as soon as the wireless industry perceived that the demand
for cellular services was growing rapidly and that the analog networks in major market
areas would quickly reach saturation. The industry also recognized that customer
demand was growing for wide-area wireless data services. Consequently, 2G systems
were designed to support a complete set of standards for all four sectors of the wireless
information network industry. As we discussed in reviewing the evolution of voiceand
data-oriented networks, there are a number of digital cellular, PCS, mobile data,
and wireless LAN standards and products that can be classified as 2G systems. In the
remainder of this section we cover each of these four categories of 2G systems in a
separate subsection.
2G Digital Cellular Systems. Table 2.5 summarizes the major 2G digital cellular
standards. There are four standards in this category: (1) GSM, the pan-European
TABLE 2.5 Second-Generation Digital Cellular Standards
System GSM IS-54 JDC IS-95
Region Europe, Asia United States Japan United States, Asia
Access method TDMA/FDD TDMA/FDD TDMA/FDD CDMA/FDD
Modulation scheme GMSK π/4-DQPSK π/4-DQPSK SQPSK/QPSK
Frequency band (MHz) 935–960 869–894 810–826 869–894
890–915 824–849 940–956 824–849
1477–1489
1429–1441
1501–1513
1453–1465
Carrier spacing (kHz) 200 30 25 1250
Bearer channels/carrier 8 3 3 Variable
Channel bit rate (kb/s) 270.833 48.6 42 1228.8
Speech coding (kb/s) 13 8 8 1–8 (variable)
Frame duration (ms) 4.615 40 20 20
digital cellular standard; (2) IS-54, which evolved into IS-136 on the North American
continent; (3) JDC in Japan; and (4) IS-95 on the North American continent.
The first three of these standards all use TDMA technology; the fourth, IS-95, uses
CDMA technology.
As in 1G analog systems, 2G systems all utilize FDD transmission and operate in
the bands from 800 to 900 MHz. The channel spacing in IS-54 and JDC is the same
as channel spacing in 1G analog systems in their respective regions, although GSM
and IS-95 use the bandwidth of multiple analog channels to form one digital channel.
GSM supports eight users in a 200-kHz digital channel; IS-95 and JDC support three
users in 30 and 25 kHz, respectively. As we explain in Chapter 11, where we discuss
access methods, the number of users that can be served by a CDMA system depends
on the acceptable quality of service, and therefore the number of users in a 1250-kHz
CMDA channel cannot be fixed theoretically. However, this number is high enough that
considering the superior voice quality achieved with CDMA, the CDMA technology
has been dominant in the planning for next-generation 3G standards.
In examining the spectrum utilization numbers for these 2G systems, one might
come to the conclusion that GSM uses 25 kHz of bandwidth for each caller, whereas IS-
95 typically uses about 10 kHz per caller, and therefore GSM supports 2.5 time fewer
calls in a given bandwidth. However, the reader should be aware that this is an illusory
conclusion, because when the network is deployed, the quality of service delivered
also depends on the frequency reuse factor and signal-to-noise interference requirements,
which will change these calculations significantly. These issues are addressed
in Chapter 11.
The channel bit rate in the GSM standard is 270 kb/s, whereas IS-54 and JDC use
48 and 42 kb/s, respectively. The higher channel bit rate in a digital cellular system
allows convenient implementation of higher data rates for data services. By assigning
several voice slots to one user on a single carrier, one can easily increase the maximum
supportable data rate for a data service offered by the network. The higher channel rate
of GSM, which utilizes eight voice slots, allows support of higher data rates, as we
discuss in Chapter 15, where we treat GPRS and EDGE mobile data services. Using a
similar argument, one may notice that the 1228.8-kb/s channel bit rate of IS-95 provides
a good framework for integration of higher data rates into the IS-95 standards. This fact
has been exploited in 3G wideband CDMA systems to support data rates up to 2 Mb/s.
Cellular standards were developed with an expectation of large cell sizes and a
large number of users per cell, which necessitates lower speech coding rates. Thus, the
speech coding techniques used in 2G systems all operate at around 10 kb/s. On the
other hand, those standards were developed initially assuming installation of mobile
phones in automobiles, where power consumption and battery life were not an issue.
The peak transmission power of mobile terminals in these standards ranges from several
hundred milliwatts up to 1 W [Pah95], and on the average they consume around
100 mW. All these systems employ central power control, which reduces battery consumption
and helps in controlling the overall interference level in the network. In
digital communication, information is transmitted in packets. The duration of a packet
frame in the transmission channel should be short enough so that the channel does not
change significantly during the transmission, and long enough that the required guardtime
gap between packets is much smaller than the length of the packet. A frame length
of around 12
to several tens of milliseconds is typically used in voice-oriented digital
cellular networks 2G PCS Systems. As we discussed in reviewing the development history of wireless
voice-oriented networks, 2G PCS standards evolved out of the 1G analog cordless
telephone industry and later merged into 3G cellular systems. Table 2.6 illustrates a
quantitative comparison of PCS and cellular industries that at its time was used to
justify the existence of two separate voice-oriented standards. The basic philosophy
was that PCS was intended for residential applications, and small cell sizes, zonal
coverage, and antennas installed on existing structures (such as utility poles). Since
PCS was not intended for high-mobility use, the complexity of the handsets and base
stations was low. These standards incorporated 32-kb/s speech coding to provide customers
with voice quality comparable to that of wireline service. Furthermore, in the
interest of achieving simpler implementation, PCS systems shared the same spectrum
in different zones, and most systems used time-division-duplex (TDD) and noncoherent
modulation techniques.
Table 2.7 provides a summary of specifications for the four major PCS standards.
CT-2 and CT-2+ were the earliest digital cordless telephone standards; PHS, which
TABLE 2.6 Quantitative Comparison of PCS and Cellular Characteristics
System Aspect PCS Cellular
Cell size 5–500 m 0.5–30 km
Coverage Zonal Comprehensive
Antenna height (m) <15 >15
Vehicle speed (km/h) <5 <200
Handset complexity Low Moderate
Base station complexity Low High
Spectrum access Shared Exclusive
Average handset power (mW) 5–10 100–600
Speech coding 32-kb/s ADPCM 7- to 13-kb/s vocoder
Duplexing Usually TDD FDD
Detection Noncoherent Coherent
TABLE 2.7 Second-Generation PCS Standards
System CT2+ DECT PHS PACS
Region Europe, Canada Europe Japan United States
Access method TDMA/TDD TDMA/TDD TDMA/TDD TDMA/FDD
Frequency band (MHz) 864–868
944–948
1880–1900 1895–1918
1850–1910
1930–1990
Carrier spacing (kHz) 100 1728 300 300, 300
Bearer channels/carrier 1 12 4 8 per pair
Channel bit rate (kb/s) 72 1152 384 384
Modulation GFSK GFSK π/4-DQPSK π/4-DQPSK
Speech coding (kb/s) 32 32 32 32
Handset Tx power (mW)
Average 5 10 10 25
Peak 10 250 80 200
Frame duration (ms) 2 10 5 2.5 later became PHP, was the first and the only one of these systems to be deployed nationwide;
and PACS is the last standard developed with this philosophy. Except for CT2+,
all of these standards were designed for operation in the 1.8- and 1.9-GHz frequency
bands, which are commonly referred to as PCS bands; all use TDMA/TDD except
PACS, which adopted frequency-division duplex (FDD) for two-way transmission.
To support voice quality comparable to that of wireline service, speech coding
at 32 kb/s is used in all of these standards. This rate is about three times higher
than the speech-coding rate used in digital cellular systems. The channel bandwidth
(1.728 MHz) in DECT is even higher than that in GSM (200 kHz), which had the
highest channel bandwidth of the TDMA digital cellular systems. This channel bandwidth
is even higher than in IS-95 (1.2288 MHz), the 2G CDMA standard. This feature
provides an advantage to DECT in supporting high-speed data connections for Internet
access.
Power consumption in PCS systems is almost one order of magnitude lower than
the power consumption in digital cellular systems because PCS systems are designed
for smaller cells. If digital cellular systems were deployed with the same cell sizes, the
average power consumption could be comparable to that of PCS systems. The modulation
techniques used for PCS standards, GFSK and DQPSK, are less bandwidth efficient
and more power efficient than are the modulation techniques used in digital cellular
systems. These modulation techniques can be implemented with simpler noncoherent
receivers. The shorter propagation time for the short-distance PCS standards allows
shorter packet frames, benefiting the voice quality despite the presence of wireless
channel impairments.
Mobile Data Services. Mobile data services provide wide-area access to packet-switched
data networks at moderate data rates. Following the success of the paging industry,
mobile data networks emerged to provide two-way transmission for longer messages.
Table 2.8 provides a comparison among a number of important mobile data services.
ARDIS and Mobitex use their own frequency bands in the region 800 to 900 MHz;
Terrestrial European Trunked Radio (TETRA) uses its own band at 300 MHz; CDPD
shares the AMPS bands and site infrastructure; and GPRS shares GSM’s complete
radio system.
Mobile Data Services
System ARDIS Mobitex CDPD TETRA GPRS
Frequency band
(MHz)
800 bands,
45-kHz
separation
935–940 869–894 380–383 890–915
896–961 824–849 390–393 935–960
Channel bit rate
(kb/s)
19.2 8.0 19.2 36 200
RF channel
spacing (kHz)
25 12.5 30 25 200
Channel access/
multiuser access
FDMA/
DSMA
FDMA/
dynamic
S-Aloha
FDMA/
DSMA
FDMA/
DSMA
FDMA/TDMA/
reservation
Modulation
technique
4-FSK GMSK GMSK π/4-QPSK GMSK
The early systems, ARDIS, Mobitex, and CDPD, were developed before the growth
in popularity of the Internet, and the dominant design criteria were coverage and cost
rather than data rate. These systems provided a wireless replacement for voiceband
modems operating at data rates up to 19.2 kb/s, which was the achievable rate of these
modems at that time. TETRA is designed for pan European civil service application
and has its own features for that purpose. GPRS supports data rates more suitable for
Internet access. The advantage of GPRS is that it is incorporated into the popular GSM
digital services, with a large number of terminals deployed all over the world. Thus,
the early mobile data systems have largely been overtaken by data services integrated
into the GSM and CDMA cellular networks.
Channel spacing used in mobile data service networks is based on the channel spacing
of cellular telephone networks, with 30- or 25-kHz bands or a fraction (12.5 kHz)
or a multiple (200 kHz) of them. These services are designed to use multiple carriers
in an FDMA format and use different versions of random access techniques such as
DSMA, BTMA, or ALOHA, discussed in Chapter 11, which deals with access methods.
Modulation techniques used in these systems are like those in digital cellular and
PCS systems.
Wireless LANs. Wireless LANs provide high-data-rate (minimum of 1 Mb/s) access in
a local area (<100 m) to wired LANs and the Internet. Today, all successful wireless
LAN products operate in the unlicensed bands. Each new product design must undergo
FCC approval, but the owner of the WLAN equipment may deploy the equipment at
will, and its operation requires no license and is not subject to further regulation.
Considering that the PCS bands had been auctioned off at very high prices, in the past
several years users have given renewed attention to the use of wireless LANs. Table 2.9
summarizes the IEEE 802.11 family of standards for wireless LAN products. The
IEEE standards include 802.11 and 802.11b operating at 2.4 GHz, 802.11a operating
at 5 GHz, and 802.11g operating at 2.4 GHz. Another extension of the IEEE family,
IEEE 802.11n, intended for even higher data rates, is still under development, and
completion is expected by late 2006.
Table 2.10 summarizes the HIPERLAN standards. Both HIPERLAN1 and 2, developed
under ETSI, operate at 5 GHz. The standardization initiatives for WLANs operating
in the 5-GHz bands led the FCC in 1997 to release the Unlicensed National
Information Infrastructure (U-NII) bands, summarized in Table 2.11.
TABLE 2.9 IEEE 802.11 Specifications
Parameter 802.11b 802.11a 802.11g
Standard approved July 1999 July 1999 June 2003
Maximum data rate 11 54 54
(Mb/s)
Modulation CCK OFDM OFDM and CCK
Data rates (Mb/s) 1, 2, 5.5, 11 6, 9, 12, 24, 48, 54 CCK: 1, 2, 5.5, 11
OFDM: 6, 9, 12, 24,
36, 48, 54
Frequencies (GHz) 2.4–2.497 5.15–5.35 2.4–2.497
5.425–5.675
5.725–5.875 HIPERLAN Standards
Parameter HIPERLAN2 HIPERLAN1
Frequency band (GHz) 5 5
PHY layer, modulation OFDM GMSK
Data rate (Mb/s) 6, 9, 12, 18, 24, 23.5
36, 54
Access method Central control,
reservationbased
access
Active contention
resolution,
priority
signaling
TABLE 2.11 Properties of U-NII bands
Band of
Operation
(GHz)
Maximum
Tx Power
(mW)
Maximum
Power with
Antenna Gain
of 6 dBi (mW)
Maximum
PSD
(mW/MHz)
Applications:
Suggested
and/or
Mandated
Other
Remarks
5.15–5.25 50 200 2.5 Restricted to
indoor
applications
Antenna must be
an integral part
of the device
5.25–5.35 250 1000 12.5 Campus
LANs
Compatible with
HIPERLAN
5.725–5.825 1000 4000 50 Community
networks
Longer range in
low-interference
(rural) environs
The 2.4-GHz products operate in ISM bands using spread-spectrum technology to
support data rates ranging from 1 to 11 Mb/s. HIPERLAN1 uses GMSK modulation
with decision feedback equalization (DFE) signal processing at the receiver and supports
rates up to 23.5 Mb/s. The IEEE 802.11a and HIPERLAN2 standards use an
OFDM physical layer to support data rates up to 54 Mb/s. The access method for all
IEEE 802.11 standards is the same and includes CSMA/CA, PCF, and RTS/CTS, which
are described in Chapter 11. The access method of HIPERLAN1 is similar to that of
802.11, but the access method for HIPERLAN2 is a voice-oriented access technique
suitable for integration of voice and data services. The IEEE 802.11 and HIPERLAN
standards can be considered as 2G wireless LANs. The 3G wireless LANs use OFDM
modulation. The IEEE 802.11g standard, approved in June 2003, operates in the 2.4-
GHz band, using DSSS and OFDM, providing data rates up to 54 Mb/s. We describe
these systems in further detail in Chapter 9, under the topic of broadband modem
technologies.